Since their earliest inception, implantable medical devices (IMDs) have continually been advanced in significant ways. Today, IMDs include therapeutic and diagnostic devices, such as pacemakers, cardioverter/defibrillators, hemodynamic monitors, neurostimulators, and drug administering devices, as well as other devices for alleviating the adverse effects of various health ailments.
As is known, modern electrical therapeutic and diagnostic devices for the heart and other areas of the body generally include an electrical connection between the device and the body. This connection is usually provided by at least one medical electrical lead. For example, a neurostimulator delivers mild electrical impulses to neural tissue using one or more electrical leads. In turn, such neurostimulation often results in effective pain relief and a reduction in the use of pain medications and/or repeat surgeries. Each electrical lead used with such devices typically takes the form of a long, generally straight, flexible, insulated conductor. At its proximal end, the lead is typically connected to a connector of the device, which also may be implanted within the patient's body. Generally, one or more electrodes are located at or near the distal end of the lead and are attached to, or otherwise come in contact with, the body. Such devices may be controlled by a physician or a patient through the use of an external programmer.
It is well known that, if not shielded sufficiently, the implanted leads of medical devices can be adversely affected when a patient is exposed to alternating electromagnetic fields. Alternating electromagnetic fields can generally stem from any of a number of radio-frequency radiation sources, e.g., magnetic resonance imaging (MRI) systems as described below. As such, if an implanted medical lead is not sufficiently shielded, electromagnetic fields can induce an electric current within a conductor of the lead. In turn, such an implanted electrical lead would act as an antenna, resulting in an electrical current that flows from the electrode of the lead and through body tissue. Because the tissue area associated with electrode contact may be very small, the current densities may be high, which can result in tissue heating that may cause damage.
There can be other limitations associated with exposing implanted leads of medical devices to electromagnetic fields and/or radio-frequency energy if the leads are not sufficiently shielded therefrom. As is known, a sudden burst of radio-frequency energy can cause an electric pulse within the lead. The medical device, as should be appreciated, can sense the imposed voltage on the lead, and in turn, may cause the device to respond inappropriately, resulting in the wrong therapy being administered to the patient at that time or in the future. For example, with respect to cardiac IMDs, inappropriate therapy modification may be one response of the IMD, which can involve changing the rate or thresholds associated with pacing pulses.
As is known, magnetic resonance imaging (MRI) is an anatomical imaging tool which utilizes non-ionizing radiation (i.e., no x-rays or gamma rays) and provides a non-invasive method for the examination of internal structure and function. For example, MRI permits the study of the overall function of the heart in three dimensions significantly better than any other imaging method. Furthermore, MRI scanning is widely used in the diagnosis of diseases and injuries to the head. Magnetic resonance spectroscopic imaging (MRSI) systems are also known and are herein intended to be included within the terminology “MRI” systems or scanners. These MRI systems can be used to give valuable diagnostic information, but also subject the patient to significant alternating electromagnetic fields and/or radio-frequency energy, which may result in one or more of the undesirable effects described above with respect to IMDs or medical devices using implanted leads.
A variety of different coverings have been used for implantable leads of medical devices to overcome the above limitations. Some coverings have involved metal or metal alloy wire being braided around the lead, thereby forming a shield having a large conductive surface area. Other coverings have involved the use of polymer-matrix composites. Such composite coverings, as opposed to metal wire coverings, are attractive due to their moldability. In addition, the composite coverings are more favorable because metal wire coverings can be prone to damage by scratching, abrasion, or wear.
Polymer-matrix composite coverings are conductive due to the presence therein of electrically conducting fillers, which can be discontinuous (e.g., such as particles or short fibers) or continuous (e.g., such as continuous fibers). As is known, even though they lack the continuity provided by continuous fillers, discontinuous fillers can just as well be used for electromagnetic shielding. Moreover, discontinuous fillers are suitable for composite fabrication by extrusion or injection molding and, if the discontinuous filler is fine enough in size, even by ink-jet printing or screen printing. Due to the lower cost and greater versatility of composite fabrication for discontinuous fillers compared to continuous fillers, discontinuous fillers have been widely used in making electrically conducting composites, especially those for electromagnetic shielding.
While polymer-matrix composites having discontinuous fillers have been used as lead coverings to reduce the effects of electromagnetic radiation, they have been found to present certain limitations, e.g., with respect to their overall shielding effectiveness. What is needed is apparatus used to reduce the potential adverse effects to medical devices, and in particular, to implantable electrical leads of the devices, when subjected to electromagnetic radiation, while further overcoming one or more of the limitations facing the discontinuous filler polymer-matrix composite lead coverings marketed to date.